Audio amplifier system



Jan; 9, 1945. I A.' A. FAYER 2,367,110

AUDIO AMPLIFIER SYSTEM I Filed March 9Q 1938 Patented .Fan. 9, 1945 sumo AMPLIFIER. SYSTEM Alfred Ayton Fayers, Heston, England, assignor to Philco Corporation, a corporation oi' Pennsylvania Application March 9, 1938, Serial No. 194,939

In Great Britain March 12, 1937 6 Claims. (or. 179-111) This invention relates to audio frequency ampliflers for radio receivers and the like, and more particularly todegenerative feedback circuits for such amplifiers. The invention provides a degenerative circult which is novelly arranged in combination with the volume control means so that the magnitude of the feedback voltage. may

- be varied in accordance with the setting of the volume control means.

In radio receivers employing degenerative feedback circuits, and particularly in the smaller or less sensitive radio receivers, it has been found desirable to reduce the degree of degenerative feedback as the volume level control of the receiver is advanced, so-that when maximum volume is required, theoverall sensitivity of the receiver will not be decreased by the effects of the degenerative feedback. On the. other hand, for volume levels less than the maximum level, it is highly desirable to take advantage of the bene fits obtainable from the use of degenerative feed- 1 back, andtherefore it is desirable to gradually increase the degree ofdegenerative feedback as the volume level is decreased. The present invention accomplishes these desirable results by means of; a novel cooperative arrangement of feedback circuit and volume control means;

The invention has for its principal object,

therefore, the provision of an improved audio frequency amplifier embodying a novel degenerative feedback control circuit.

Another object of this invention is to provide 'novel degenerative feedback control means arranged soas to be controllable by the volume control meansof the system.

A further object of the invention is to provide a novel volume control device for controlling both a degenerative feed-back signal and an audio signal, both of which are applied to the audio channel.

Other objects and features of the lnvention will appear hereinafter. v

In'the accompanying drawing, the single fig- I ure is a schematic illustration of an audio amplifler embodying the invention.

Referring to the drawing, there is shown an audio frequency amplifier system comprising a first amplifier including the triode I, followed by; a final amplifier 2, output transformer 3, and loud-speaker L. S. The source of input signal E representative of any suitable source of audio frequency signal, such as the usual second detector of the well-known superheterodyne circult, a phonograph .pick-up, or other source of signal. A potentiometer lserves to control the amplitude of the input signal supplied to the triode l. Connected across the secondary winding of the output transformer is a voltage divider consisting of the serially-connected resistors 5 and 6, the lower extremity of resistor 6 being con nected to ground. The cathode of the triode I is connected 'to the common point between the resistors 5 and 6 by means of the conductor 1.

Since the ohmic resistanceof the voice coil of the loud-speaker is comparatively low, the parallel combination of the resistors 5 and 6 serves as a cathode bias resistor for the triode I in addition to the functions described hereinafter The upper extremity of the potentiometer is connectedto the upper extremity-of resistor 5 by -means of the connection 8 which includes the serially-connected resistors 9 and Ill.

Disregarding the several condensers for the present, it will be seen that there are two voltage feedback paths to the triode i, one via the lead 1 to the cathode and the other through the serially-connected resistors 9 and ill to the control grid. A portion of the volume control potentiometer 4 may or may not beincluded in this latter feedback path depending upon the setting of the movable arm ll. The values of the several resistances are such that when the arm is moved to the upper end of the potentiometer,

the voltages fed back to the grid and the cathode are equal both in phase and magnitude. For this condition no feedback voltage will appear between the grid and cathode of triode l, and hence I degeneration cannot take place. However, when the potentiometer arm is moved toward ground, this balance of potentials is disturbed and degeneration takes place. It will be seen that the resistances 5, 6, 9 and I0 together with the potentiometer 4, form a bridge circuit, to one diagonal of which the feedback voltage is applied whilst the grid-cathode input of the triode (at maximum gain setting) is connected across the other diagonal of the bridge. The resistances are so proportioned that the bridge is balanced in the condition of maximum gain, and no feedback voltage is applied to the triode ,I. When gain is decreased, however, the bridge is unbalanced and a negative feedback voltage is applied to the input of the triode I, the amplitude of the voltage depending upon the extent of unbalance of the bridge. In this way, as the output level is diminished, and the receiver has more than the required sensitivity, the quality of reproduction is improved due to the increasing degenerative. feedback.

Considering the system in a somewhat different manner, the connection" 8 including the resistors 9 and I is a regenerative connection while the connection I is a degenerative connection. When the arm H is at the upper end of the potentiometer, the regenerative feedback voltage and the degenerative feedback voltage cancel one another, due to the voltage drop across the resistors 9 and I0 and that across the pocult means connected between said output circuit and said input circuit, said last two mentioned circuit means comprising a plurality of impedances arranged cooperatively with said potentiometer to form an electrical bridge, the electrical balance of said bridge being dependent on the adjustment of said potentiometer, and means for varying the degenerative action for different frequencies within the frequency response range of said amplifier.

2. In an audio amplifying system, an audio amplifier having an input circuit and an output circuit, said amplifier including a space discharge device having a control grid and a cathode, a potentiometer connected in said input circuit and having a movable arm connected to said control grid for varying the amplitude of signals supplied to said amplifier, a plurality of impedances connected serially across said output circuit, means feedback voltage is-applied to .the input of triode l, which voltage increases as the arm II is moved downward. In other words, the voltage of the grid with respect to ground decreases, whil the voltage of the cathode with respect to ground for connecting an extremity'of one of said impedances to an extremity of said potentiometer, means for connecting an extremity of another of said impedances to the other extremity of said potentiometer, and means for connecting said cathode to a common point of said impedances intermediate the said extremities thereof.

remains constant. the result being that an increasing degenerative voltage is established be tween the grid and cathode. At the same time, the volume level of the input signal is.decreased by the same amount that the resultant degenerative voltage increases. Thus, the input signal amplitude and the effective degenerative voltage are varied oppositely.

By making the various circuits frequency discriminating, certain desirable effects may be obtained. Thus, bass compensation may be obtained by connecting the resistor I2 and condenser l3 across the lower part of the potentiometer 4, although it is to be understood that this feature may be omitted if desired. A will be apparent, this capacitive by-pass will attenuate the high frequencies. The degenerative feedback may be made more effective at low than at high frequencies by the incorporation of the shunt condenser l4 across the resistor 6 as shown. The condenser I5 is employed in order to balance the system. The overall result is then as follows: At maximum gain, n0 modification of the input frequency characteristic occurs and the resultant response is that due to the existing circuit conditions. As the gain is decreased, however, the high frequency response, due to the negative feedback, becomes proportionally greater, while the bass compensator makes the bass notes more predominant. These two eiTects overlap to some extent and the overall response can be made such that the middle frequenciesbecome attenuated more rapidly than the extreme frequencies.

Although a specific preferred form of the in-. vention has been illustrated and described for the purpose of disclosure, it will be understood that the invention is not limited thereto but is capable of various embodiments and modifications. The scope of the invention is defined only by the appended claims.

I claim:

1. In an audio amplifying system, an audio amplifier having an input circuit and an output circuit, means for supplying signal energy to said input circuit, a potentiometer connected to said input circuit for varying the amplitude of signals supplied to said amplifier, regenerative circuit means connected between said output circuit and said amplitude varying means, degenerative cir- 3. In an audio amplifying system, an audio amplifier having an input circuit and an output circuit. said amplifier including a space discharge device having a control grid and a cathode, a potentiometer connected in said input circuit and having a movable'arm connected to said control grid for varying the amplitude of signals supplied to saidamplifier, a plurality of impedances connected serially across said output circuit, a connection including at least one impedance element between an extremity of one of said impedances and the high potential end of said potentiometer, means for connecting an extremity of another of said impedances to the low potential end of said potentiometer, and means for connecting said cathode to a common point of said impedances intermediate thesaid extremities thereof.

4. In an audio amplifying system, an audio amplifier having an input circuit and an output circuit, said amplifier including a space discharge device having a control grid and a cathode, a potentiometer connected in said input circuit and having a movable arm connected to said control grid for varying the amplitude of signals supplied to said amplifier, a plurality of impedances connected serially across said output circuit, a connection including at least one impedance element between an extremity of one of said impedances and the high potential end of said potentiometer, means for connecting an extremity of another of said impedances to the low potential end of said potentiometer, means for connecting said cathode to a common point of said impedances intermediate the said extremities thereof, and frequencyselective means arranged cooperatively with sail. potentiometer and with the said other impedance.

5. An audio frequency amplifier having an input circuit and an output circuit, means comprising a potentiometer for supplying signal energy of adjustable magnitude to the input of said amplifier, a first feedback path between said output and said input circuits for supplying a regenerative signal to said amplifier. a second feedback path between said output and said input circuits for supplying a degenerative signal to said amplifler,-means including said potentiometer, said input and output circuits, and said feedback paths for constituting a Wheatstone bridge circuit 'of said second path.

circuits, said bridge being substantialiy'baianoed at only one adjustment of said potentiometer, whereby at said one adjustment the signal fed back by way of said. first path is substantially cancelled by the opposing signal fed back by way 6. In combination, a variable-gain variable feedback amplifier including a Wheatstone bridge and an amplifying device having input terminals and output terminals, said bridge having a first pair of diagonal points and a second pair oi di agonal points, means for supplying feedback volt- .ai voltage mtween'one oi age mm said output terminals to said first pair of diagonal points, and means for supplying a said second pair of diagonal ints and another point of the bridge, the input terminals of said amplifying device being connected to the other of said second pair of diagonal points and an adjustable contact on an arm oi said bridge, the feedback voltage supplied to said input terminals by way of said bridge being reduced substantially to zero for one adjustment of said adjustable contact. 

